Follow:
AdminCP -> Styles & Templates -> Style Manager
For each style you wish to affect the change, follow:
Edit Templates
Locate the group "Postbit Templates", double-click and then open the template "postbit_legacy."
Locate the opening tag:
HTML Code:
<div class="postbody">
And change it to read:
HTML Code:
<div class="postbody" style="<vb:if condition="($thread['postuserid'] == $post['userid']) AND in_array($thread['forumid'], array(XX, YY, ZZ))">background: #ffeb90</vb:if>">
Replace "XX, YY, ZZ" with the actual list of forumids of the forums you want this change to affect, and you can change the background to another color, I simply used the default shade of yellow we get when we select posts for inline moderation.